refactor(catalog): reduce the category taxonomy to a supplier label

Categories are kept only as the label each supplier carries (suppliers.category_id
+ the material_categories list it points at). Everything that treated category as
a shared key across materials, inventory, quotes and unit conversions is removed —
including two features that were never populated (0 rows on every volume).

Removed (backend):
- routers category_attributes, category_ref_units, unit_conversions + registration
- ?category_id= filters on /materials, /inventory and /quotes (the /suppliers one
  stays — it filters suppliers by their own label)
- GET /inventory/materials/options: required a category and had no caller
- the material-payload category derivation (expand_service.category_by_material,
  the inventory serializer lookup) and the material category on /pm/on-the-way;
  the wire keys stay, now always null, so the payload shape is unchanged
- startup DDL: DROP the empty category_attributes / category_reference_units /
  unit_conversions tables (idempotent) and their CREATE statements; seed.py
  truncate list updated

Removed (frontend):
- the category-keyed conversion path: refUnitForCategory / findConversion /
  refConverted, the categoryUnits + unitConversions store slices, api clients,
  hooks and the "≈ converted quantity" line on quote detail and project pages
  (it needed rows in those tables to render, and there were none)
- material.supplier_category and its read sites (the edit modal grew one
  yesterday to restore labels; the label lives on the supplier now)
- the inventory page's category chips, the row filter and the «دسته» column,
  plus the lookup's categories fetch and catName helper

Kept: supplier label + its list and picker, /suppliers?category_id=, the global
standard_conversions feature and the units page (material_units conversions).

Verified against a restore of the production dump: startup DDL drops exactly the
three tables, suppliers keep their label (3/3), material_categories still serves
its 8 rows, the removed endpoints 404, and materials/inventory/quotes/units/
standard-conversions all return 200 with quote line items intact.

backend/app/db/init_db.py

@@ -18,28 +18,9 @@ logger = logging.getLogger(__name__)
 
 _DASHBOARD_DDL = [
     # One reference unit per category (units.html «واحد مرجع هر دسته»)
-    """
-    CREATE TABLE IF NOT EXISTS category_reference_units (
-        id          UUID        NOT NULL DEFAULT gen_random_uuid() PRIMARY KEY,
-        category_id UUID        NOT NULL UNIQUE REFERENCES material_categories(id) ON DELETE CASCADE,
-        ref_unit    TEXT        NOT NULL,
-        updated_at  TIMESTAMPTZ NOT NULL DEFAULT now()
-    )
-    """,
     # Brand-scoped / category-scoped conversions: 1 from_unit = factor × to_unit.
     # UNIQUE must treat brand NULL as a concrete value (PG treats NULLs as distinct).
     """
-    CREATE TABLE IF NOT EXISTS unit_conversions (
-        id          UUID           NOT NULL DEFAULT gen_random_uuid() PRIMARY KEY,
-        category_id UUID           NOT NULL REFERENCES material_categories(id) ON DELETE CASCADE,
-        brand       TEXT,
-        from_unit   TEXT           NOT NULL,
-        to_unit     TEXT           NOT NULL,
-        factor      NUMERIC(18,6)  NOT NULL,
-        updated_at  TIMESTAMPTZ    NOT NULL DEFAULT now()
-    )
-    """,
-    """
     CREATE UNIQUE INDEX IF NOT EXISTS uq_unit_conversion
         ON unit_conversions (category_id, COALESCE(brand, ''), from_unit, to_unit)
     """,
@@ -413,6 +394,22 @@ async def ensure_v26(engine: AsyncEngine) -> None:
     """)
 
 
+async def ensure_drop_category_tables(engine: AsyncEngine) -> None:
+    """v2.7 follow-up: the category taxonomy is reduced to a supplier label.
+
+    These three tables backed features that were never populated (0 rows on every
+    volume): per-category reference units and conversion factors (the
+    per-reference-unit price line) and the category -> attribute mapping. Dropping
+    them is idempotent and loses no data. material_categories itself stays — every
+    supplier carries one as its label.
+    """
+    async with engine.begin() as conn:
+        await conn.execute(text(
+            "DROP TABLE IF EXISTS category_attributes, category_reference_units, unit_conversions CASCADE"
+        ))
+    logger.info("category taxonomy tables dropped (attributes / ref units / conversions)")
+
+
 async def ensure_audit_table(engine: AsyncEngine) -> None:
     """v2.7: general activity trail (audit_log). Idempotent; safe on any volume."""
     ddl = """